High Lockout
AL21 on Vokera Unica 24HE

Your boiler has detected an issue with an external safety device or thermostat connection, causing it to stop running for safety.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

External device fault (UHT/CPA)

What causes this fault?

This fault occurs when an external safety component, such as an underfloor heating limit thermostat, detects an unsafe temperature and breaks the electrical circuit to the boiler. It can also be caused by a loose connection or a fault within the internal wiring harness that prevents the control board from seeing a complete safety loop.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

Find a Gas Safe engineer
Search the official register →

Related fault codes

AL20View →AL34View →

Related guides you might find useful

Boiler won't turn on — 9-step checklist

Work through these before the call-out.

Read →

Boiler keeps cutting out

Lockout fault with a reset-needed pattern.

Read →

Yellow or orange flame — CO safety

Safety-critical — turn off, read this.

Read →